Coating composition comprising a polyisocyanate and a polyol

ABSTRACT

The invention relates to a non-aqueous coating composition comprising a. a polyisocyanate, b. a polyol having an average functionality of more than 3 hydroxyl groups per molecule, c. a metal based curing catalyst for the addition reaction of isocyanate groups and hydroxyl groups, and d. a mercapto carboxylic acid, wherein the coating composition does not contain a carboxylic acid wherein the carbonyl group of the carboxylic acid is in conjugation with π-electron system.

REFERENCE TO RELATED APPLICATION(S)

This application is the U.S. National Phase of PCT/EP2009/063644 filedon Oct. 19, 2009, and claims the benefit of U.S. Provisional ApplicationNo. 61/107,862 filed on Oct. 23, 2008.

The invention relates to a coating composition comprising apolyisocyanate, a polyol, a metal based catalyst for the additionreaction of isocyanate groups and hydroxyl groups, and athiol-functional compound. The invention also relates to a kit of partsfor preparation of the coating composition and to the use of thecomposition.

WO 2007/020270 describes a coating composition comprising apolyisocyanate, a polyol, a metal based catalyst for the additionreaction of isocyanate groups and hydroxyl groups, a thiol-functionalcompound, and a carboxylic acid wherein the carbonyl group of thecarboxylic acid is in conjugation with a π-electron system.

GB 2188327 A relates to polyol/polyisocyanate coating compositions andmore particularly to a catalyst system effective therefor. The catalystsystem comprises the reaction product of a metal catalyst and a molarexcess of a complexing agent. The complexing agent is a mercaptocompound or a polyphenol. This document discloses coating compositionscontaining a tri-functional polyol, a polyisocyanate, dibutyl tindilaurate, and various mercaptans. Mercaptans with carboxylfunctionality are reported to shorten the pot life of the formulation.

Although coatings with acceptable properties can be prepared from theknown composition, there is an ongoing need for further improvement ofthe balance of pot life, appearance properties, curing rate, andhardness of the cured coatings, in particular in cases where a high orvery high non-volatile content, i.e. a low content of volatile organiccompounds (VOC), is required. Furthermore, in practice coatings areapplied under various conditions of ambient temperature and atmosphericmoisture. An improved balance of desirable properties should be achievedunder various application conditions. Using a lower amount of curingcatalyst decreases the risk of coating defects caused by entrappedsolvents and/or gases. However, it leads to longer drying times anddecreased hardness of the coatings as well, with the risk of saggingand/or dirt pick-up. Longer drying times are undesirable in view of thehigh throughput of coating operations. Furthermore, low VOC coatingcompositions comprising a high curing catalyst load to obtain a shortdrying time suffer from foam stabilization in the drying coating,leading to pinholes in the dried coating layer. Pinholes detract fromthe appearance and durability of coating layers. Thus, a very goodbalance of high curing rate, long pot life, low VOC content, and goodappearance and hardness of the cured coating cannot be achieved with theknown coating compositions.

Accordingly, the invention seeks to provide a coating composition havinga favourable balance of properties, i.e. a low content of volatileorganic solvent at application viscosity, a high cure speed, and a longpot life, leading to coatings with good appearance properties, inparticular a low susceptibility to pinholes, and good hardness. Inaddition, the coating composition should also provide cured coatingsexhibiting other properties required for motor vehicle exteriorfinishes, such as flexibility, scratch resistance, gloss, durability,and resistance to chemicals and UV radiation.

The invention now provides a coating composition comprising

-   -   a. a polyisocyanate,    -   b. a polyol having an average functionality of more than 3        hydroxyl groups per molecule,    -   c. a metal based curing catalyst for the addition reaction of        isocyanate groups and hydroxyl groups, and    -   d. a mercapto carboxylic acid,        wherein the coating composition does not contain a carboxylic        acid wherein the carbonyl group of the carboxylic acid is in        conjugation with a π-electron system.

The coating composition of the invention provides a favourable balanceof properties, i.e. a low content of volatile organic solvent atapplication viscosity, a high cure speed, and a long pot life, leadingto coatings with good appearance properties, in particular a lowsusceptibility to pinholes, and good hardness. In addition, the coatingcomposition also provides cured coatings exhibiting other propertiesrequired for motor vehicle exterior finishes, such as flexibility,scratch resistance, gloss, durability, and resistance to chemicals andUV radiation.

Suitable isocyanate-functional crosslinkers for use in the coatingcomposition are isocyanate-functional compounds comprising at least twoisocyanate groups. Preferably, the isocyanate-functional crosslinker isa polyisocyanate, such as an aliphatic, cycloaliphatic or aromatic di-,tri- or tetra-isocyanate. Examples of diisocyanates include1,2-propylene diisocyanate, trimethylene diisocyanate, tetramethylenediisocyanate, 2,3-butylene diisocyanate, hexamethylene diisocyanate,octamethylene diisocyanate, 2,2,4-trimethyl hexamethylene diisocyanate,dodecamethylene diisocyanate, ω,ω′-dipropylether diisocyanate,1,3-cyclopentane diisocyanate, 1,2-cyclohexane diisocyanate,1,4-cyclohexane diisocyanate, isophorone diisocyanate,4-methyl-1,3-diisocyanatocyclohexane, trans-vinylidene diisocyanate,dicyclohexyl methane-4,4′-diisocyanate (Desmodur® W), toluenediisocyanate, 1,3-bis(isocyanato-methyl)benzene, xylylene diisocyanate,α,α,α′,α′-tetramethyl xylylene diisocyanate (TMXDI®),1,5-dimethyl-2,4-bis(2-isocyanatoethyl)benzene,1,3,5-triethyl-2,4-bis(isocyanatomethyl)benzene,4,4′-diisocyanato-diphenyl, 3,3′-dichloro-4,4′-diisocyanato-diphenyl,3,3′-diphenyl-4,4′-diisocyanato-diphenyl,3,3′-dimethoxy-4,4′-diisocyanato-diphenyl, 4,4′-diisocyanato-diphenylmethane, 3,3′-dimethyl-4,4′-diisocyanato-diphenyl methane, anddiisocyanato-naphthalene. Examples of triisocyanates include1,3,5-triisocyanatobenzene, 2,4,6-triisocyanatotoluene,1,8-diisocyanato-4-(isocyanatomethyl)octane, and lysine triisocyanate.Adducts and oligomers of polyisocyanates, for instance biurets,isocyanurates, allophanates, uretdiones, urethanes, and mixtures thereofare also included. Examples of such oligomers and adducts are the adductof 2 molecules of a diisocyanate, for example hexamethylene diisocyanateor isophorone diisocyanate, to a diol such as ethylene glycol, theadduct of 3 molecules of hexamethylene diisocyanate to 1 molecule ofwater (available under the trademark Desmodur N of Bayer), the adduct of1 molecule of trimethylol propane to 3 molecules of toluene diisocyanate(available under the trademark Desmodur L of Bayer), the adduct of 1molecule of trimethylol propane to 3 molecules of isophoronediisocyanate, the adduct of 1 molecule of pentaerythritol to 4 moleculesof toluene diisocyanate, the adduct of 3 moles ofm-α,α,α′,α′-tetramethyl xylene diisocyanate to 1 mole of trimethylolpropane, the isocyanurate trimer of 1,6-diisocyanatohexane, theisocyanurate trimer of isophorone diisocyanate, the uretdione dimer of1,6-diisocyanatohexane, the biuret of 1,6-diisocyanatohexane, theallophanate of 1,6-diisocyanatohexane, and mixtures thereof.Furthermore, (co)polymers of isocyanate-functional monomers such asα,α′-dimethyl-m-isopropenyl benzyl isocyanate are suitable for use.

The coating composition further comprises a polyol having an averagefunctionality of more than 3 hydroxyl groups per molecule. These may bemonomers, oligomers, polymers, and mixtures thereof. Oligomers andpolymers are generally preferred. Examples of suitable polymers includepolyester polyols, polyacrylate polyols, polycarbonate polyols,polyurethane polyols, melamine polyols, and mixtures and hybridsthereof. Such polymers are generally known to the skilled person and arecommercially available. Suitable polyester polyols, polyacrylatepolyols, and mixtures thereof are for example described in Internationalpatent application WO 96/20968 and in European patent application EP0688840 A. Examples of suitable polyurethane polyols are described inInternational patent application WO 96/040813.

Further examples include hydroxy-functional epoxy resins, alkyds, anddendrimeric polyols such as described in International patentapplication WO 93/17060.

In one embodiment, the polyol has an average functionality of at least 4hydroxyl groups per molecule, or at least 5 hydroxyl groups permolecule. Generally, the polyol has an average functionality of at most20 hydroxyl groups per molecule. In other embodiments, the polyol has anaverage functionality of at most 15, or at most 10, hydroxyl groups permolecule.

The coating composition can additionally comprise latenthydroxy-functional compounds such as compounds comprising bicyclicorthoester, spiro-orthoester, spiro-ortho silicate groups, or bicyclicamide acetals. These compounds and their use are described inInternational patent applications WO 97/31073, WO 2004/031256, and WO2005/035613, respectively.

In one embodiment, the coating composition comprises a polyester polyolhaving an average functionality of more than 3 hydroxyl groups permolecule and a polyacrylate polyol having an average functionality ofmore than 3 hydroxyl groups per molecule.

As mentioned above, the coating composition of the invention alsocomprises a metal based catalyst for the addition reaction of isocyanategroups and hydroxyl groups. Such catalysts are known to the skilledperson. The catalyst is generally used in an amount of 0.001 to 10weight-%, preferably 0.002 to 5 weight-%, more preferably in an amountof 0.01 to 1 weight-%, calculated on the non-volatile matter of thecoating composition. Suitable metals in the metal based catalyst includezinc, cobalt, manganese, zirconium, bismuth, and tin. Tin, bismuth andzirconium based catalysts have been found to perform particularly well.It is preferred that the coating composition comprises a tin basedcatalyst. Well-known examples of tin based catalysts are dimethyl tindilaurate, dimethyl tin diversatate, dimethyl tin dioleate, dibutyl tindilaurate, dioctyl tin dilaurate, and tin octoate.

As mentioned above, the coating composition comprises a mercaptocarboxylic acid as essential ingredient. Mercapto carboxylic acids arecarboxylic acids which also have a mercapto group in the molecule.Without wishing to be bound by any theory, it is believed that mercaptocarboxylic acids are capable of forming a chelate type complex with themetal atoms of the metal based catalyst. Such complex formation isfurther believed to cause a temporary deactivation of the metal basedcatalyst. It has further been found that particularly favourable resultscan be obtained with 2-mercapto-carboxylic acids and 3-mercaptocarboxylic acids. These mercapto carboxylic acids have been found tolead to longer open times of the applied coatings, leaving more time forentrapped gas bubbles to escape. These mercapto carboxylic acids arebelieved to act as bidentate ligands for the metal atoms of the metalbased curing catalyst. Examples of suitable mercapto carboxylic acidsinclude methylthioglycolate, 2-mercaptoacetic acid, mercaptosuccinicacid, cysteine, 3-mercaptopropionic acid, 2-mercaptopropionic acid,11-mercaptoundecanoic acid, and 2,3-dimercaptosuccinic acid.

The mercapto carboxylic acid is generally present in an amount of 0.1 to18 weight-%, preferably 0.2 to 10 weight-%, more preferably in an amountof 0.3 to 5 weight-%, calculated on the non-volatile matter of thecoating composition. The actual amount of mercapto carboxylic aciddepends on the type and amount of metal based catalyst employed, on thethiol equivalent weight of the mercapto carboxylic acid, and on thedesired property profile of the coating composition. In some embodimentsit can be beneficial to use the mercapto carboxylic acid in such anamount that the composition comprises a molar excess of thiol groupsover the metal atoms of the metal based catalyst.

As mentioned above, the mercapto carboxylic acids are believed to act asbidentate ligands for the metal atoms of the metal based curingcatalyst, forming a cyclic complex. It has been found that instead ofincluding the metal based curing catalyst and the mercapto carboxylicacid as separate components in the coating composition, it is alsopossible to pre-mix the mercapto carboxylic acid and the metal basedcuring catalyst, which upon mixing form a reaction product which can assuch been included in the coating composition. Therefore, in anotherembodiment, the invention also relates to a non-aqueous coatingcomposition comprising

-   -   a. a polyisocyanate,    -   b. a polyol having an average functionality of more than 3        hydroxyl groups per molecule, and    -   c. the reaction product of        -   i) a metal based curing catalyst for the addition reaction            of isocyanate groups and hydroxyl groups and        -   ii) a mercapto carboxylic acid,            wherein the coating composition does not contain a            carboxylic acid wherein the carbonyl group of the carboxylic            acid is in conjugation with a π-electron system.

In the coating composition according to the invention the equivalentratio of isocyanate-functional groups to hydroxyl groups suitably isbetween 0.5 and 4.0, preferably between 0.7 and 3.0, and more preferablybetween 0.8 and 2.5. Generally, the weight ratio of hydroxy-functionalbinders to isocyanate-functional crosslinker in the coating composition,based on non-volatile content, is between 85:15 and 15:85, preferablybetween 70:30 and 30:70.

The liquid coating composition may be used and applied without avolatile diluent, in particular when low molecular weight binders,optionally in combination with one or more reactive diluents, are used.Alternatively, the coating composition may optionally comprise avolatile organic solvent. Preferably, the coating composition comprisesless than 500 g/l of volatile organic solvent based on the totalcomposition, more preferably less than 480 g/l, and most preferably 420g/l or less. The non-volatile content of the composition, usuallyreferred to as the solid content, preferably is higher than 50 weight-%based on the total composition, more preferably higher than 54 weight-%,and most preferably higher than 60 weight-%.

Examples of suitable volatile organic diluents are hydrocarbons, such astoluene, xylene, Solvesso 100, ketones, terpenes, such as dipentene orpine oil, halogenated hydrocarbons, such as dichloromethane, ethers,such as ethylene glycol dimethyl ether, esters, such as ethyl acetate,ethyl propionate, n-butyl acetate or ether esters, such as methoxypropylacetate or ethoxyethyl propionate. Also mixtures of these compounds canbe used.

If so desired, it is possible to include one or more so-called “exemptsolvents” in the coating composition. An exempt solvent is a volatileorganic compound that does not participate in an atmosphericphotochemical reaction to form smog. It can be an organic solvent, butit takes so long to react with nitrogen oxides in the presence ofsunlight that the Environmental Protection Agency of the United Statesof America considers its reactivity to be negligible. Examples of exemptsolvents that are approved for use in paints and coatings includeacetone, methyl acetate, parachlorobenzotrifluoride (commerciallyavailable under the name Oxsol 100), and volatile methyl siloxanes. Alsotertiary butyl acetate is being considered as an exempt solvent.

In addition to the components described above, other compounds can bepresent in the coating composition according to the present invention.Such compounds may be binders and/or reactive diluents, optionallycomprising reactive groups which may be crosslinked with the aforesaidhydroxy-functional compounds and/or isocyanate-functional crosslinkers.Examples of such other compounds are ketone resins, aspargyl acidesters, and latent or non-latent amino-functional compounds such asoxazolidines, ketimines, aldimines, diimines, secondary amines, andpolyamines. These and other compounds are known to the skilled personand are mentioned, int. al., in U.S. Pat. No. 5,214,086.

The coating composition may further comprise other ingredients,additives or auxiliaries commonly used in coating compositions, such aspigments, dyes, surfactants, pigment dispersion aids, levelling agents,wetting agents, anti-cratering agents, antifoaming agents, antisaggingagents, heat stabilizers, light stabilizers, UV absorbers, antioxidants,and fillers.

The coating composition of the invention can be applied to anysubstrate. The substrate may be, for example, metal, e.g., iron, steel,and aluminium, plastic, wood, glass, synthetic material, paper, leather,or another coating layer. The other coating layer can be comprised ofthe coating composition of the current invention or it can be adifferent coating composition. The coating compositions of the currentinvention show particular utility as clear coats, base coats, pigmentedtop coats, primers, and fillers. When the coating composition of theinvention is a clear coat, it is preferably applied over a colour-and/or effect-imparting base coat. In that case, the clear coat formsthe top layer of a multi-layer lacquer coating such as typically appliedon the exterior of automobiles. The base coat may be a water borne basecoat or a solvent borne base coat. The coating compositions are suitablefor coating objects such as bridges, pipelines, industrial plants orbuildings, oil and gas installations, or ships. The compositions areparticularly suitable for finishing and refinishing automobiles andlarge transportation vehicles, such as trains, trucks, buses, andairplanes.

The applied coating composition can be cured at ambient temperature, forexample between about 10° C. and about 40° C. Alternatively, curing canbe carried out at elevated temperature, for example in the range ofabout 40° C. to about 80° C. Curing at a temperature of about 60° C. hasbeen found to be particularly favourable. If so desired, the coatingcomposition may be oven cured. Alternatively, curing may be supported by(near) infrared radiation. Before curing at elevated temperature theapplied coating composition may optionally be subjected to a flash-offphase to evaporate at least a part of the volatile solvent present inthe coating composition.

It is to be understood that the term coating composition as used hereinalso includes its use as adhesive composition.

As is usual with coating compositions comprising a hydroxy-functionalbinder and an isocyanate-functional crosslinker, the compositionaccording to the invention has a limited pot life. Therefore, thecomposition is suitably provided as a multi-component composition, forexample as a two-component composition or as a three-componentcomposition. Therefore, the invention also relates to a kit of parts forpreparation of the coating composition comprising

-   a) a binder module comprising a polyol having an average    functionality of more than 3 hydroxyl groups per molecule,-   b) a crosslinker module comprising a polyisocyanate, and-   c) optionally, a diluent module comprising a volatile organic    solvent,    wherein a metal based catalyst for the addition reaction of    isocyanate groups and hydroxyl groups and a mercapto carboxylic acid    are distributed, individually or in combination, over one or more of    the modules.

In a preferred embodiment, the metal based catalyst for the additionreaction of isocyanate groups and hydroxyl groups and the mercaptocarboxylic acid are distributed, individually or in combination, overone or more of modules a) and c). Providing the components of thecomposition in the form of a kit of parts has the additional advantagethat the components can be provided in the required molar ratios. Hence,the risk of errors caused by incorrect mixing ratios of the individualcomponents is minimized.

EXAMPLES

Unless stated otherwise, the properties of the liquid coatingcompositions or the resulting coating films were determined as describedbelow.

The DINC4 viscosity was measured in a DIN Flow cup (number 4). Theviscosity is given in seconds.

The volatile organic content (VOC) was calculated theoretically from thesolvent content of the coating compositions.

The Enamel Hold Out (EHO) was determined as the total visual appearance.Each sample was rated for visual appearance on a scale of 1 to 10(1=very bad, 10=excellent). The determination takes into account gloss,wrinkling, flow, and image clarity/distinctness of image. The averagenumber will give the EHO. The pinhole sensitivity was determinedvisually. Each sample was rated on a scale of 1 to 10 at comparablelayer thicknesses of the coating (1=very bad, 10=excellent).

The warm tackiness was determined manually after 5 minutes at the end ofthe 60-65° C. cure cycle. Each sample was rated on a scale of 1 to 10 atcomparable layer thicknesses of the coating (1=very bad, 10=excellent).

The open times were determined with a BK drying recorder. The dryingrecorder measurements were done to determine the drying rate and theopen time of freshly applied coatings at the indicated temperature. Withthe drying recorder four different phases can be distinguished:

-   Phase 1: The needle track reflows to a closed film.-   Phase 2: The needle track is regular and does not reflow to a closed    film. The coating is damaged down to the substrate.-   Phase 3: The needle track is irregular, with incidental damage down    to the substrate.-   Phase 4: The needle track is regular again, but the coating shows    only superficial damage. At the end of stage 4 no damage is observed    at all.

The coating layers used for the drying recorder experiments were appliedwith a K-Controle coating apparatus.

The Persoz hardness was measured according to ISO 1522.

The polyester polyol with a functionality of 2.8 which was used in theexamples was prepared as described in international patent publicationWO 2007/0020269, p. 22, Example E2.

The polyacrylate polyols used in the Examples were in an analogous wayas described in international patent publication WO 2007/0020269, p. 19,l. 16-p. 21, Examples A1-A4.

Abbreviations:

-   2-MPA 2-Mercaptopropionic acid-   3-MPA 3-Mercaptopropionic acid-   TGA Thioglycolic acid-   E2-MPA Ethyl-2-mercaptopropionate-   E3-MPA Ethyl-3-mercaptopropionate-   DBTL Dibutyl tin dilaurate-   NVC Non-volatile content-   Tolonate HDT LV Aliphatic polyisocyanate HDI trimer ex Rhodia ppmc-   Desmodur XP 2511 Aliphatic polyisocyanate HDI trimer ex Bayer    Material Science Ag-   BYK 331 Polydimethyl siloxane polyether (modified) additive, ex BYK    Chemie GmbH-   BYK 355 Polyacrylate solution, ex BYK Chemie GmbH-   Vestanat T1890 E Cycloaliphatic polyisocyanate, ex Evonik Degussa    GmbH-   BYK 392 Acrylate copolymer additive, ex BYK Chemie GmbH-   Exxsol D30 Naphta (petroleum), ex Exxon Mobil Chemical Company-   Butylcellosolve® acetate 2-Butoxyethyl acetate, ex DOW Chemical    Company Ltd.-   PTMP Pentaerythritol tetrakis(3-mercaptopropionate)-   Tolonate HDT LV 2 Aliphatic polyisocyanate HDI, ex Rhodia ppmc-   SBP 140/165 Naphta (petroleum), ex Shell Chemicals Europe B.V.-   Byk 320 Polymethyl-alkylsiloxane, polyether (modified) additive, ex    BYK Chemie GmbH-   Tinuvin 1130 Benzotriazole derivative mixture, ex Ciba Specialty    Chemicals-   Tinuvin 292 Hindered amine light stabilizer, ex Ciba Specialty    Chemicals-   T_(g) Glass transition temperature (° C.)-   f(OH) Average OH functionality

Example 1

Six clearcoat compositions were prepared by mixing the components in theweight proportions as indicated in Table 1. The molar ratio ofisocyanate groups to hydroxyl groups was adjusted to 1.25:1. The clearcoat compositions of Table 1 all contained the following additives:

0.6 parts by weight Tinuvin 1130

1.1 parts by weight Tinuvin 292

0.1 part by weight BYK 331

0.1 part by weight BYK 355

TABLE 1 Clear coat composition E1.1 E1.2 E1.3 E1.4 E1.5 E1.6 Acrylicpolyol (f(OH) = 6.2; 55.6 55.6 55.6 55.6 55.6 55.6 NVC = 65%, T_(g) =29° C.) Polyester polyol (f(OH) = 2.8; 19.9 19.9 19.9 19.9 19.9 19.9 NVC= 100%; T_(g) = −59° C.) n-butylacetate 16.1 16.1 16.1 16.1 16.1 16.1Ethyl-ethoxy-propionate 0.9 0.9 0.9 0.9 0.9 0.9 SBP 140/165 4.5 4.5 4.54.5 4.5 4.5 Tolonate HDT LV/ 44.9 44.9 44.9 44.9 44.9 44.9 Desmodur XP2511 Ethyl-ethoxy-propionate 9.6 9.6 9.6 9.6 9.6 9.6 n-butylacetate 9.69.6 9.6 9.6 9.6 9.6 DBTL 0.22 0.22 0.22 0.22 0.22 0.22 Methyl-amylketone6.2 5.3 6.6 5.9 6.7 6.0 SBP 140/165 8.9 8.9 8.9 8.9 8.9 8.9 BYK-392 0.490.49 0.49 0.49 0.49 0.49 n-butylacetate — 0.87 — 0.87 — 0.872-Mercaptopropionic acid — — 0.24 0.24 0.12 0.12 PTMP 0.28 0.28 — — — —Benzoic Acid 0.28 0.28 — — — — DBTL — 0.1 — 0.1 — 0.1 VOC (g/l) ≦420≦420 ≦420 ≦420 ≦420 ≦420 Clear coat composition E1.1 E1.2 E1.3 E1.4 E1.5E1.6 Acrylic polyol (f(OH) = 6.2; 55.6 55.6 55.6 55.6 55.6 55.6 NVC =65%, T_(g) = 29° C.) Polyester polyol (f(OH) = 2.8; 19.9 19.9 19.9 19.919.9 19.9 NVC = 100%; T_(g) = −59° C.) n-butylacetate 16.1 16.1 16.116.1 16.1 16.1 Ethyl-ethoxy-propionate 0.9 0.9 0.9 0.9 0.9 0.9 SBP140/165 4.5 4.5 4.5 4.5 4.5 4.5 Tolonate HDT LV/ 44.9 44.9 44.9 44.944.9 44.9 Desmodur XP 2511 Ethyl-ethoxy-propionate 9.6 9.6 9.6 9.6 9.69.6 n-butylacetate 9.6 9.6 9.6 9.6 9.6 9.6 DBTL 0.22 0.22 0.22 0.22 0.220.22 Methyl-amylketone 6.2 5.3 6.6 5.9 6.7 6.0 SBP 140/165 8.9 8.9 8.98.9 8.9 8.9 BYK-392 0.49 0.49 0.49 0.49 0.49 0.49 n-butylacetate — 0.87— 0.87 — 0.87 2-Mercaptopropionic acid — — 0.24 0.24 0.12 0.12 PTMP 0.280.28 — — — — Benzoic Acid 0.28 0.28 — — — — DBTL — 0.1 — 0.1 — 0.1 VOC(g/l) ≦420 ≦420 ≦420 ≦420 ≦420 ≦420

The clear coat compositions were based on two different DBTL blockingsystems, namely PTMP/benzoic acid (E1.1, E1.2) and 2-MPA (E1.3 to E1.6).In samples E1.5 and E1.6 the amount of 2-MPA was half that in samplesE1.3 and E1.4. In samples E1.2, E1.4 and E1.6 the amount of DBTL wasincreased. 2-MPA was used in equal molar quantities compared to benzoicacid. PTMP was used in equal molar SH quantities.

TABLE 2 Viscosity development of samples E1.1 to E1.6 at 20° C. SampleT_(0 min) T_(15 min) T_(30 min) T_(45 min) E1.1 16.1 17.3 23.4 38.1 E1.216.1 17.6 25.9 46.5 E1.3 16.1 16.1 16.3 16.7 E1.4 16.0 16.0 16.5 17.3E1.5 15.9 15.9 16.6 18.5 E1.6 15.8 15.8 17.1 20.0

From Table 2 it can be inferred that the viscosity increase of samplesE1.1 and E1.2 is the fastest in time. Samples E.1.3 and E1.4 have theslowest viscosity increase in time, followed by samples E.1.5 and 1.6.

The blocking behaviour of 2-MPA is better than the PTMP/benzoic acidcatalyst blocking system. Even with higher amounts of DBTL and half theamount of 2-MPA (sample E1.6) the viscosity increase is slower than insample E1.1. A slower viscosity increase indicates a longer pot life. Inaddition, the use of 2-MPA over PTMP/benzoic acid is preferred from aneconomic point of view.

Drying recorder experiments at 60° C. and 20° C. are in line with theviscosity increase experiments of Table 2. Sample E1.3 has the longestopen time (the slowest viscosity increase). A long open time isadvantageous for air and solvent escape from the coating layer. SamplesE1.1, E1.3, and E1.5 are directly comparable.

TABLE 3 Drying recorder results at 60° C. Sample Phase 1 (cm) Phase 2(cm) Phase 3 (cm) E1.1 1.3 0.2 1.7 E1.3 3.4 0.5 2.3 E1.5 3.1 0.4 1.8

TABLE 4 Drying recorder results at 20° C. Sample Phase 1 (cm) Phase 2(cm) Phase 3 (cm) E1.1 1.8 0.4 1.7 E1.3 4.7 2.5 5.7 E1.5 3.1 2.7 5.7

Example 2

A clear coat composition was prepared by mixing the components in theweight proportions as indicated in Table 5. The molar ratio ofisocyanate groups:hydroxyl groups was adjusted to 1.03:1. The clearcoatcompositions of Table 5 below contained the following additives:

0.6 parts by weight Tinuvin 1130

1.0 part by weight Tinuvin 292

0.1 part by weight BYK 331

1.0 part by weight BYK 392

TABLE 5 Clear coat compositions E2.1 E2.2 E2.3 E2.4 Acrylic polyol(f(OH) = 6.2; 52.3 52.3 52.3 52.3 NVC = 75%, T_(g) = −12° C.) Polyesterpolyol (f(OH) = 2.8; 19.9 19.9 19.9 19.9 NVC = 100%, T_(g) = −59° C.)n-butylacetate 25 25 25 25 Ethyl-ethoxy-propionate 0.9 0.9 0.9 0.9 DBTL0.2 0.2 0.2 0.2 Tolonate HDT LV 2 33.6 33.6 33.6 33.6 Vestanat T1890 E12.8 12.8 12.8 12.8 Methyl-isoamyl-ketone 3.5 3.5 3.5 3.5 n-butylacetate3.5 3.5 3.5 3.5 n-butylacetate 14.3 14.1 13.7 14.4 Xylene 5.2 5.2 5.25.2 Butylcellosolve ® acetate 5.2 5.2 5.2 5.2 2-Mercapto-ethanol 0.292-MPA 0.39 Benzoic acid 0.45 PTMP 0.45 0.45 VOC gr/L ≦420 ≦420 ≦420 ≦420

The clear coat compositions were based on 4 different DBTL blockingcomponents, namely 2-MPA (E2.1, according to the invention), PTMP (E2.2,comparative), PTMP/benzoic acid (E2.3, comparative), and2-mercapto-ethanol (E2.4, comparative). The catalyst blocking agentswere used in equal molar quantities or, in the case of PTMP, replaced byequal molar SH quantities.

The clearcoat compositions were sprayed by a coating robot, 30 minutesafter mixing, at 25° C. and 75-80% relative humidity. Aluminium panelswere used as a substrate. The substrates were pre-coated successivelywith a chromate-free etch primer, a non-sanding filler, and a waterborne metallic base coat. Once the coating had been applied, the panelswere immediately transferred to an oven kept at 60° C. withoutintermediate flash-off time.

TABLE 6 Visual/physical characteristics of the clear coat compositionsPersoz hardness Sample EHO Flow Warm tackiness (after 20 hours) E2.1 8 87 38 E2.2 6-7 7 8 37 E2.3 7 7 8 35 E2.4 2 2 8 35

Sample E2.1 according to the invention was judged to be the best amongthe four samples. The Enamel Hold Out and the flow were better than inthe case of the comparative Examples. The warm tackiness of sample E2.1was rated to be 7. The warm tackiness of samples E2.2-E2.4 was rated tobe 8.

The formulation E2.1, containing 2-MPA, has the longest open time, atboth 20° and 60° C. A long open time is advantageous for the flow of acoating and for air and solvent escape from the coating layer. A propersolvent and air escape is advantageous for the EHO.

TABLE 7 Open time at 20° C. Phase 1 (cm) Phase 2 (cm) Phase 3 (cm) E2.15.4 0.9 2.2 E2.2 3.0 0.4 1.0 E2.3 3.4 0.4 1.9 E2.4 2.9 0.4 0.9

TABLE 8 Open time at 60° C. Phase 1 (cm) Phase 2 (cm) Phase 3 (cm) E2.12.3 0.3 2.0 E2.2 1.6 0.3 1.5 E2.3 1.8 0.3 2.0 E2.4 1.3 0.3 1.3

Example 3

Five clear coat compositions were prepared by mixing the components inthe weight proportions as indicated in Table 9.

The clear coat compositions of Table 9 below contained the followingadditives:

0.6 parts by weight Tinuvin 1130

1.1 parts by weight Tinuvin 292

0.1 part by weight BYK 331

0.1 part by weight BYK 355

TABLE 9 Clear coat composition E3.1 E3.2 E3.3 E3.4 E3.5 Acrylic polyol(f(OH) = 6.2; 55.6 55.6 55.6 55.6 55.6 NVC = 65%, T_(g) = 29° C.)Polyester polyol (f(OH) = 2.8; 19.9 19.9 19.9 19.9 19.9 NVC = 100%;T_(g) = −59° C.) n-butylacetate 16.1 16.1 16.1 16.1 16.1Ethyl-ethoxy-propionate 0.9 0.9 0.9 0.9 0.9 SBP 140/165 4.5 4.5 4.5 4.54.5 Tolonate HDT LV/ 44.9 44.9 44.9 44.9 44.9 Desmodur XP 2511Ethyl-ethoxy-propionate 9.6 9.6 9.6 9.6 9.6 n-butylacetate 9.6 9.6 9.69.6 9.6 DBTL 0.22 0.22 0.22 0.22 0.22 2-MPA 0.24 E2-MPA 0.31 3-MPA 0.24E3-MPA 0.31 TGA 0.21 SBP 100-140 9 9 9 9 9 Methyl amyl ketone 6 6 6 6 6VOC g/l ≦420 ≦420 ≦420 ≦420 ≦420

The clear coat compositions were based on 5 different DBTL blockingcomponents, namely 2-MPA (E3.1, according to the invention), E2-MPA(E3.2, comparative), 3-MPA (E3.3, according to the invention), E3-MPA(E3.4, comparative), and TGA (E3.5, according to the invention). Allblocking agents were used in equal molar quantities, and are thereforedirectly comparable. Below, the viscosity development at 20° C. ofsamples E3.1 to E3.5 measured by DIN cup 4 is summarized

TABLE 10 Viscosity increase (pot life) DC 4, 20° C. Sample T_(0 min)T_(15 min) T_(30 min) T_(45 min) T_(60 min) T_(75 min) E3.1 15.6 16.016.4 16.7 17.1 17.6 E3.2 15.6 16.0 16.8 18.5 22.5 32.9 E3.3 15.5 16.016.3 17.3 18.2 20.9 E3.4 15.5 16.0 17.0 19.3 25.3 44.3 E3.5 15.7 16.016.3 16.7 17.3 18.0

From Table 10 it is evident that sample E3.1 has the lowest viscosityafter 75 minutes. The compositions according to the invention containing2-mercapto-carboxylic acid (E3.1 and E3.5) have a lower viscosity after75 minutes compared to the comparative formulations containing the ethylester of 2-mercapto-carboxylic acid functionality (E3.2). The same trendof viscosity development is observed between samples E3.3 (slowest) andE3.4 (fastest). The samples E3.1, E3.3, and E3.5 demonstrate that athiol and carboxylic acid functionality in close proximity is beneficialfor good catalyst blocking behaviour. This phenomenon can most likely beexplained by bidentate-coordination of the thiol-carboxylic acid ligandtowards the catalyst.

Drying recorder experiments at 60° C. show the same observations as inthe case of the pot life experiments; the formulation containing 2-MPAhas the slowest viscosity development.

TABLE 11 Drying recorder at 60° C.: Sample Phase 1 (cm) Phase 2 (cm)Phase 3 (cm) E3.1 3.1 0.6 1.4 E3.2 1.6 0.3 1.2 E3.3 1.8 0.2 2.1 E3.4 1.30.3 0.9 E3.5 2.6 0.5 1.1

Example 4

Using the Lesonal™ HS 420 top coat line two car body resprays wereperformed according to the technical data sheet of the product. TheLesonal HS 420 top coat line is a ≦420 gr/l solvent bornepolyester/polyacrylate polyol-isocyanate top coat system, especiallydesigned for the car refinish market. Polyester polyol: f(OH)=4.4;NVC=80%; t_(g)=−2° C. Polyacrylate polyol: f(OH)=4.1; NVC=74%, t_(g)=20°C. The red colour RAL 3020 was selected as A-component and prepared fromthe Lesonal™ HS 420 top coat toner assortment. Organic pigmented (e.g.red/blue) A-components are known to have high pinhole sensitivity. TheLesonal™ HS420 hardener was used as the hardener for both samples. Onetop coat ready-to-spray mixture was finished with sample E4.1(comparative), the other sample was finished with sample E4.2 (accordingto the invention)

TABLE 12 Sample E4.1 E4.2 2-Mercaptopropionic acid 1.5 BYK-392 4.25Methyl-isoamyl-ketone 24 22 Methyl-amyl-ketone 24 10.25Ethyl-ethoxy-propionate 14.25 Exxsol D30 37.75 Butylcellosolve acetate ®10 Acetyl acetone 50 Byk 320 2.0

The two car body resprays were performed in a spray booth at 35° C. and25% humidity. The car bodies used were similar to a normal sized 5-doorpassenger car. The two samples were sprayed 15 minutes after thepreparation of the ready-to-spray compositions. The top coat was appliedin two layers, with 5-10 minutes flash-off between the layers. After thetop coat had been applied, there was no flash-off time and the appliedcoating was cured at 60-65° C. for 35 minutes.

TABLE 13 Sample 4 E4.1 E4.2 Pinholes 5 9 EHO 7 8 Sprayability firstlayer 7 7 Sprayability second layer 6 7 Warm tackiness 8 7 Flow 7 8

The results in Table 13 demonstrate improvements in pinhole sensitivity,EHO, sprayability, and flow of sample E4.2 over E4.1.

Example 5

Application of 420 g/l top coats in an industrial manner (e.g.airless/electrostatic apparatus) can be problematic due too significantamounts of air entrapment in top coats. Air entrapment causes pinholesduring curing. This example shows the advantage of thinner sample E5.2according to the invention over comparative Example E5.1 based on knowncatalyst blocking compounds. These two samples were used together withthe Sikkens™ Autocoat BT LV 351™ Topcoat and Hi Flo hardener. TheSikkens™ Autocoat BT LV 351™ is a solvent borne 420 g/lpolyester/polyacrylate polyol-isocyanate top coat system. Polyesterpolyol: f(OH)=4.4; NVC=80%; t_(g)=−2° C. Polyacrylate polyol: f(OH)=4.1;NVC=74%, t_(g)=20° C. The ready-to-spray mixture was prepared accordingto the technical data sheet of the product. The red colour Rat 3020 wasselected as the A-component and prepared from the top coat tonerassortment of Sikkens™ Autocoat BT LV 351™ Organic pigmented top coats(e.g. red/blue) are known to have relatively high pinhole sensitivity.Application of the two ready-to-spray mixtures was performed on steelpanels precoated with sanding filler. Subsequently the two samples wereapplied with a Graco airless spraying apparatus and immediately cured at60° C. for 30 min. No flash off time was used.

TABLE 14 Sample E5.1 E5.2 2-Mercaptopropionic acid 1.5 BYK-392 2.0 4.5Ethyl-ethoxy-propionate 45.5 36.5 Butylcellosolve ®-acetate 25 SBP140/165 28.0 30 DBTL 0.3 0.3 n-Butylacetate 2.7 1.35 Xylene 1.35 PTMP1.5 Acetylacetone 20.0

TABLE 15 Example E5.1 E5.2 Pinholes 4 9 Warm tackiness 8 7

The results in Table 15 clearly demonstrate the improvements in pinholesensitivity of coating E5.2 according to the invention. The warmtackiness indicates the curing is almost on the same level as incomparative Example E5.1. Hence, it has been demonstrated that thebalance of properties has been improved.

Example 6

Example 6 demonstrates the influence of the functionality of the polyolused in the coating composition. Example E6.1 is an example according tothe invention, containing a acrylic polyol having an averagefunctionality of 6.2 hydroxyl groups. Example E6.2 is a comparativeExample wherein the acrylic polyol of Example E6.1 was replaced with anacrylic polyol having an average functionality of 3.0 hydroxyl groups.The other properties of the acrylic polyols used in these Examples, inparticular glass transition temperature and molecular weight, were thesame. The amount if acrylic polyols in Examples E6.1 and E6.2 wasselected such that the total amount of hydroxyl groups was the same inboth compositions. The amount of additives was selected such that theirproportion on non-volatile content was the same in both compositions.Hence, any differences in the coating compositions and the coatings canbe attributed to the difference in functionality. The components of thecompositions are summarized, in parts by weight, in Table 16.

TABLE 16 Clearcoat composition E6.1 E6.2 Acrylic polyol (f(OH) = 6.2;55.6 NVC = 65%, OH number = 140 Mg KOH/g Polyester polyol (f(OH) = 2.8;19.9 19.9 NVC = 100%; T_(g) = −59° C. OH number = 303 mg KOH/g Acrylicpolyol f(OH) = 3.0 117.8 NVC = 63.8% OH number = 67.3 n-butylacetate16.1 21.9 Ethyl-ethoxy-propionate 0.9 0.9 SBP 140/165 4.5 4.5 TolonateHDT LV/Desmodur XP 2511 44.9 44.9 Ethyl-ethoxy-propionate 9.6 9.6n-butylacetate 9.6 9.6 DBTL 0.20 0.28 Methylamyl ketone 6.6 6.6 SBP140/165 8.9 8.9 Tinuvin 292 1.1 1.4 BYK-355 0.1 0.14 BYK-331 0.1 0.14BYK-392 0.5 0.7 2-Mercaptopropionic acid 0.24 0.33 VOC (g/L) ≦420 ≦420

In Table 17 the viscosity development at 20° C. of samples E6.1 and E6.5measured by DIN cup 4 is summarized.

TABLE 17 Viscosity increase (pot life) DC 4, 20° C. Sample T₀ min T₁₅min T₃₀ min T₄₅ min E6.1 15.3 15.8 16.0 16.4 E6.2 18.0 19.2 19.7 20.1

From Table 17 it can be inferred that sample E6.1 according to theinvention has a lower starting viscosity than comparative sample E6.2.furthermore, the viscosity of sample E6.2 increases faster. In can beconcluded that clearcoat sample E6.1 according to the invention can belonger processed can be longer processed that comparative sample E6.2.

The drying rate of applied coatings from samples E6.1 and E6.2 at 20° C.and at 60° C. was determined using a BK drying recorder. The results aresummarized in Table 18.

TABLE 18 Sample Phase 1 (cm) Phase 2 (cm) Phase 3 (cm) Drying recorderresults at 20° C. E6.1 7 7 5.1 E6.2 4.8 20.0 nd Drying recorder resultsat 60° C. E6.1 2.9 0.6 2.9 E6.2 3.0 3.9 3.8

The overall drying time of clearcoat sample E6.1 according to inventionis shorter then E6.2, both at 20° C. and 60° C. degrees Celsius. This isparticularly surprising, as the viscosity increase in the pot of sampleE6.1 is slower, as demonstrated above.

The clearcoat compositions were sprayed by a coating robot, 20 minutesafter mixing, at 22° C. and 45% humidity. Steel panels were used asubstrate. The substrates were pre-coated successively with a sandingfiller and a water borne, deep black, basecoat. Once the coating hadbeen applied, the panels were left for five minutes flash off timebefore being transferred to an oven kept at 60° C. The clearcoat panelswere cured for 40 minutes and left for 24 hours before being judged.

TABLE 19 Visual characteristics of the clear coats Sample EHO SaggingPinholes E6.1 9-10 9-10 9-10 E6.2 8-9  8-9  6-7 

Form table 19 it can be inferred that clearcoat formulation E6.1 hasbetter visual properties and less defects compared to sample E6.2.Sample E6.1 according to the invention was rated better for enamel holdout and sagging. A significant difference was observed in solvent pop(pinhole) sensitivity. Comparative clear coat E6.2 was judged on average6-7 (on a scale of ten), while sample E6.1 was judged on average a 9-10.

From the results of Example 6 it can be concluded that sample E.6.1according to the invention exhibits a slower viscosity increase, i.e.longer potlife, faster drying, and better visual properties than thecomparative sample E6.2. The only difference between these samples isthe functionality of the polyol used in both samples. Hence, theimproved properties of sample E6.1 can be attributed to the use of apolyol having a average functionality of more than 3 hydroxyl groups permolecule.

The invention claimed is:
 1. A non-aqueous coating compositionconsisting essentially of: (a) an aliphatic or cycloaliphaticpolyisocyanate, (b) a polyol having an average functionality of morethan 3 hydroxyl groups per molecule, wherein the polyol is selected frompolyester polyols, polyacrylate polyols, polycarbonate polyols,polyurethane polyols, melamine polyols, and mixtures thereof, (c) ametal based curing catalyst for the addition reaction of isocyanategroups and hydroxyl groups, based on a metal selected from the groupconsisting of tin, bismuth, zirconium, and mixtures thereof and (d) amercapto carboxylic acid, wherein the coating composition does notcontain a carboxylic acid wherein the carbonyl group of the carboxylicacid is in conjugation with a π-electron system.
 2. The coatingcomposition according to claim 1, wherein the catalyst for the additionreaction of isocyanate groups and hydroxyl groups is based on tin. 3.The coating composition according to claim 1, wherein the amount ofmetal based curing catalyst is in the range of 0.001 to 10 weight-%,calculated on the non-volatile matter of the composition.
 4. The coatingcomposition according to claim 1, wherein the mercapto carboxylic acidis selected from the group consisting of 2-mercapto-carboxylic acids,3-mercapto-carboxylic acids, and mixtures thereof.
 5. The coatingcomposition according to claim 1, wherein the amount of mercaptocarboxylic acid is in the range of 0.1 to 18 weight-%, calculated on thenon-volatile matter of the composition.
 6. The coating compositionaccording to claim 1, wherein component b) comprises a polyester polyoland a polyacrylate polyol.
 7. The coating composition according to claim1, wherein the composition further comprises a volatile organic solvent.8. The coating composition according to claim 7, wherein the amount ofvolatile organic solvent does not exceed 420 g/l of coating composition.